Ecological Responses to Climate Change on the Antarctic Peninsula

The Base of the Food Chain

The western coast of the Antarctic Peninsula is a highly productive ecosystem, but it is also closely attuned to the rhythms of the physical environment and thus is vulnerable to disruption. Of particular importance are fluctuations in sea ice, both seasonally and from year to year.

A bloom of phytoplankton in spring and summer depends on the annual cycle of the ice. Many single-celled plants overwinter in pockets of liquid within the ice. In the spring melt, the phytoplankton are released and exposed to increased sunlight, stimulating their growth and causing the bloom. Diatoms—single-celled phytoplankton with siliceous shells—are best adapted to the sea ice margin and dominate the blooms over the continental shelf of the western peninsula. The diatoms are the preferred food for Antarctic krill, a key link in the food chain; krill pass energy and nutrients captured by phyto­plankton up to penguins, seals and whales.

As regional warming reduces both the extent and the duration of sea ice cover, changes are already evident at most levels of the food web. In some areas, for example, there are indications that diatoms are being replaced by cryptophytes—smaller phytoplankton lacking mineral shells. If such a shift becomes widespread, it will surely have observable effects at higher trophic levels.

Krill, which are crustaceans that resemble shrimp, are the principal Antarctic zooplankton. They are highly dependent on sea ice; without it they cannot complete their life cycle and breed successfully. Juvenile krill congregate under the ice, browsing on the algae growing in fissures and using the ice habitat as a refuge from predators. As sea ice declines, the krill habitat is shrinking in space and time.

In addition to krill, two other groups are major components of the zooplankton: salps and pteropods. Salps are gelatinous, transparent organisms that look a little like jellyfish although they are actually chordates, primitive relatives of the vertebrates. They pump water through mucus feeding webs and skim off the adhering food particles. Individual salps can be as large as 10 centimeters, and colonies are meters long. They can be voracious predators, clearing large volumes of water of all particles larger than a few micrometers. Salps are pelagic organisms, usually inhabiting offshore waters with lower plankton concentrations (their feeding nets become clogged in blooms); they are carried onto the continental shelf by intrusions of the circumpolar current. Salps have few predators, which makes them a dead end in the food chain.

Pteropods are gastropod mollusks, sometimes called sea butterflies; they are swimming pelagic snails with calcium carbonate shells. Like salps, pteropods commonly feed with mucus nets, but they are herbivores, grazing on phytoplankton. Moreover, unlike salps, they have predators and thus participate in the food chain.

As sea ice declines and intrusions of offshore warm water increase in frequency and volume, the various kinds of zooplankton respond differently. A large-scale decline in krill populations has been under way for decades, although changes in sea ice may not be the only cause. Increasing predation could also be a factor, since the regulation of whaling has allowed a slow recovery of krill-eating whale species. But the ongoing reductions in sea ice are expected to have further adverse effects on krill. In contrast, salps may be increasing over the western peninsular continental shelf, in response to changes in ice and water properties. This replacement of krill by salps has potentially grave consequences for an Antarctic food web that is highly dependent on krill as food for larger predators, including penguins.

Shelled pteropods are threatened by another factor related to climate change: ocean acidification. As the level of carbon dioxide in the atmosphere increases, the ocean absorbs some of the excess. The CO
2
lowers the
p
H, and the acidified water dissolves the carbonate shells of mollusks, corals and other organisms. The consequences of acidification could be particularly intense for the shelled pteropods of the Southern Ocean, with detectable effects by late in the present century.

In summary, it appears that current climatic trends are likely to be detrimental to crustacean and molluscan members of the zooplankton, while favoring the "gelatinous" organisms, including the salps as well as tunicates and a few other groups. Similar changes have already happened or are currently in progress for other reasons throughout the world's oceans.

The shifting balance between krill and salps will affect still another planktonic community: bacteria and other prokaryotic micro­organisms. But it remains unclear how the bacteria will respond to population changes at higher levels in the food chain. Elsewhere in the world, rising populations of gelatinous zooplankton appear to reroute organic matter into the bacterial biomass. Whether this "microbialization" will also happen in cold Antarctic waters is not yet known.

Ironically, atmospheric carbon dioxide—the main culprit in global warming—may in turn be susceptible to influence by ecosystem changes in the Southern Ocean. Planktonic organisms take up CO
2
, converting it into carbon-rich organic compounds, some of which ultimately fall to the sea floor and are withdrawn from active circulation. The effect of various marine population shifts on this biological carbon pump is not known. An abundance of salps could increase carbon sequestration, because salps excrete large, rapidly sinking fecal aggregates. Bacteria, on the other hand, break down complex carbon compounds and release CO
2
. If this microbial respiration increases, oceanic CO
2
storage may be reduced, creating a positive feedback loop: CO
2
induces warming, which decreases the CO
2
capacity of the oceans, bringing further warming.